In wood processing machines used to process logs into chips, it has been common for the machines to include a plurality of knives mounted either on the face of a rotary disc or on the surface of a rotating drum. The logs are then guided into the disc or drum so that the knives can cut through the wood. With knives used in the prior art, it is preferable to orient the log at an angle of about 371/2-38 degrees with respect to the path of travel of the knives through the wood in order to minimize the power required to drive the knives across the grain of the wood. When using a disc type machine, the log should be fed at an angle with respect to the disc while feeding it into the blades. This arrangement requires a large amount of space for the wood processing apparatus.
When using a rotating drum type apparatus equipped with knives of the prior art, the log can be angled or can be fed straight toward the drum. However, in the latter case the log must be fed into the bottom portion of the rotating drum, as shown in U.S. Pat. No. 2,710,635, in order to take advantage of cutting the wood at the more efficient 38 degree angle. This requires a very large drum relative to the log diameter since only a small portion of the circumferential surface provides the proper angle for cutting.
The knives conventionally utilized in such known wood processing machines generally have a linear cutting edge formed by double grinding a metal blank to form a sharp edge between the back surface of the knife and an end face. Such a primary edge generally extends parallel to the surface of the disc or drum. Some of the knives in the prior art have wing edges that extend laterally from the side of the primary cutting edge and are angled from the plane of the primary edge in order to cut another side of the chip to provide a more uniformly shaped chip. Examples of this type of knife are found in U.S. Pat. Nos. 3,219,076 and 3,559,705.
The knives of the prior art machines have generally failed to produce wood chips of as uniform a width, grain length and thickness as is desired in many industries. For example, in the manufacture of chemical wood pulp for use in making paper, it is important that the wood chips employed be of as nearly uniform dimensions as possible, since the rate of penetration of the cooking liquors for digesting the wood, that is, for converting it into pulp, is dependent in part on the dimensions of the chips to be digested. It has also been recognized that conventional wood chippers as employed for the production of chips used in the manufacture of pulp can damage the wood fibers and thus result in lowered pulp quality.
In U.S. Pat. No. 4,161,972, logs are fed directly into the surface of a disc chipper. However, the knife shown therein makes a major cut along the grain of the wood, and makes only a very small cut equal to the smallest dimension of the chip (typically 1/4 inch) across the grain. The angled cutting edge of the knife of this patent is angled only about 7-10 degrees from the surface of the disc. The knife thus is not operating at an efficient angle, and is limited to a very small cut to prevent power requirements from becoming excessive. The number of knives required to remove a layer of wood from the end of a log using this apparatus is excessive. The patentee teaches against using his concepts in a drum chipper.
The prior art thus has lacked a chipper which permits logs to be fed end-on into a knife mounting surface without requiring an excessive amount of power to cut directly across the grain, or an excessive number of small knives.